The present invention relates to improvements in a wire electrical discharge machining apparatus which is capable of effectively promoting the cooling of a wire electrode and improving the machining speed.
Referring to FIGS. 5 to 8, a description will be given hereafter of the configuration and operation of a conventional wire electrical discharge machining apparatus.
FIG. 5 shows an overall configuration of mechanical portions, in which reference numeral 101 denotes a bed as a base of machine, and numeral 102 denotes an X-axis table. The X-axis table 102 is supported by an X-axis guide 103 on the bed 101, and is driven in the X-direction by an unillustrated X-axis motor through an X-axis ball screw 104. Numeral 105 denotes a table for fixing a workpiece 25, which is fixed on the X-axis table 102. Numeral 106 denotes a processing tank for storing a working fluid. Numeral 107 denotes a column for supporting a Z-axis unit 117, and a lower arm 108 is fixed thereto. A lower guide 109 is attached to a distal end portion of this lower arm 108. Numeral 118 denotes an upper guide, which is fixed to a distal end portion of the Z-axis unit 117. Numeral 111 denotes a Y-axis guide on the bed 101 which supports the column 107. The column 107 is driven in the Y-direction by a Y-axis motor 113 through a Y-axis ball screw 112. Numeral 114 denotes a wire collector for supporting a roller 115 for collecting a wire electrode 119, and the collected wire electrode 119 is accommodated in a collecting box 116. Numeral 120 denotes a wire bobbin, numeral 121 denotes a pad disposed underneath the bed 101, and numeral 122 denotes a leveling bolt for adjusting inclination.
FIG. 6 is a cross-sectional view illustrating the configuration of the lower guide 109. The lower guide 109 is fixed to a distal end of the lower arm 108 through an insulating plate 2, and is mainly comprised of a nozzle 6, an electric supply die 16, a lower wire guide holder 12, a guide supporting plate 23, and a lower block 3. The lower block 3 includes a roller 19 which functions to convert the direction of the wire electrode, and has a wire inlet 3a and a wire outlet 3b which are tapered. A collection pipe 20 is connected at the wire outlet 3b. The guide supporting plate 23 incorporates the electric supply die 16, and also incorporates a withdrawing plate 17 for withdrawing the electric supply die 16. The electric supply die 16 is fixed by a holding plate 15. Numeral 18 denotes a lower auxiliary guide, and the lower auxiliary guide 18, together with the lower wire guide holder 12, presses the wire electrode 119 against the electric supply die 16 so as to supply electricity to the wire electrode 119.
The nozzle 6 is a portion for jetting the working fluid, and the working fluid is supplied through a pipe 9 as a high-pressure fluid. Numeral 14 denotes a spring, and 13 denotes a holding plate for the nozzle 6. The nozzle 6 during machining is raised while compressing the spring 14, is stopped by the holding plate 13, and is returned downward when the working fluid ceases to be supplied. As a result, when machining is not being effected, the nozzle is lowered, thereby preventing its useless contact with the workpiece. A rectifying plate 22 has the function of rectifying the turbulence of the working fluid inside the nozzle, and a multiplicity of small holes 22a are provided therein. Numeral 10 denotes the wiring, which is connected to the guide supporting plate 23 formed of an electrically conductive material, so as to supply machining electric power from an unillustrated machining power supply to the electric supply die 16. Numeral 24 denotes a lower transporting-current jetting hole, to which a pipe from the outside is connected and which functions to transport the wire electrode 119 in the collection pipe 20 to the collection roller 115 (see FIG. 5), and is used mainly during the initial setting of the wire electrode 119. Numeral 26 denotes a machining gap between the workpiece 25 and the wire electrode 119, and this gap is referred to as the gap between the electrodes. The working fluid jetted from the nozzle 6 is supplied to the gap 26 between electrodes. The principal functions of the working fluid are, among others, to discharge the machining sludge produced during machining and to prevent the overheating of the wire electrode, and the working fluid is an essential element in preventing the disconnection of the wire electrode during machining.
FIG. 7 is a cross-sectional view illustrating the configuration of the upper guide 118. An attaching plate 225 fixes the upper guide 118 to the Z-axis unit 117 and is formed of an insulating material. An upper block 226 has a passage 226a, and an upper auxiliary guide 229 for guiding the wire electrode 119 is disposed on top of the upper block 226. An electric supply die 46 is accommodated in the interior of the upper block 226, and is pressed toward the wire electrode 119 by a pressing plate 230. The arrangement provided is such that the insertion and withdrawal of the electric supply die 46 are made possible by a withdrawing plate 47. An upper wire guide holder 236 is fixed to a lower end of the upper block 226, and a wire guide 236a is fixed to a distal end portion of the upper wire guide holder 236. Further, a housing 234 is provided in such a manner as to cover the upper wire guide holder 236, and a nozzle 232 is disposed on the outer side thereof. A jet nozzle 233, which is supported in such a manner as to be vertically movable through a spring 235, is accommodated inside the housing 234.
When the working fluid is supplied to a jet pipe 238, the jet nozzle 233 moves downward by its pressure while pressing the spring 235, and jets out a jet stream 239 through a jetting hole 233a in the jet nozzle 233. The wire electrode 119 passes through the interior of the jet stream 239, and is guided to the lower guide located therebelow. During normal machining, the working fluid is supplied from a working fluid pipe 237, and the working fluid jets out from a jetting hole 232a in the nozzle 232, and is supplied to the workpiece 25. In addition, during normal machining, the jet nozzle 233 is retracted upward by the spring 235. Numeral 221 denotes a cooling hole through which the working fluid is guided into the interior of the upper wire guide holder 236 to cool the wire electrode 119 during machining. Numeral 234a denotes a fixed throttle which is used to rectify the disturbance of the working fluid supplied from the working fluid pipe 237.
The wire electrical discharge machining apparatus is cooled by the working fluid because a large electric current flows across the contacting portions of the wire electrode and the electric supply die, and the temperature of these portions becomes high, possibly resulting in the disconnection of the wire electrode. The cooling working fluid is supplied to the wire passage 226a from the cooling hole 221 provided in the upper wire guide holder 236 by making use of the back pressure within the nozzle 232. The working fluid rises upward from the cooling hole 221 through the interior of the wire passage 226a, passes the contacting portions of the wire electrode 119 and the electric supply die 46, passes the upper auxiliary guide 229, and is discharged to the outside. Thus, as the working fluid passes, cooling is effected by absorbing Joule heat produced in the contacting portions of the wire electrode and the electric supply die.
Next, referring to FIG. 8, a description will be given of the arrangement of the wire electrode and a machining groove during machining. It is assumed that machining is being effected while maintaining a fixed gap in the direction toward the machining/advancing direction in the drawing. Jet streams 240 jet out from the upper and lower nozzles as indicated by the arrows, come into contact with each other substantially in the vicinity of the center in the vertical direction of the workpiece 25, and flow toward a groove 26b located rearwardly in the machining direction.
To increase the machining speed, it is necessary to increase the working electric current and to promote the cooling of the contacting portions of the wire electrode and the electric supply die. However, there is a limit to the cooling method which makes use of the back pressure within the nozzle, as shown in FIG. 7. Namely, if the workpiece and the nozzle are dissociated from each other and the back pressure drops, cooling becomes insufficient, with the result that there occur problems such as the occurrence of disconnection of the wire electrode and the occurrence of loading due to the wear sludge of the wire electrode caused by the temperature rise at the contacting portions between the wire electrode and the electric supply die. Further, in the conventional configuration, a multiplicity of small-diameter holes (the small-diameter holes 22a in the lower guide in FIG. 6 and the fixed throttles 234a in the upper guide in FIG. 7) are provided inside the nozzles for the purpose of rectification. Hence, pressure loss has been very large in these portions, and the cooling capacity between the electrodes has therefore been lowered.
In addition, if the machining speed is increased, the amount of machining sludge which is discharged increases, and when the production and discharge of the machining sludge fail to balance, the gap between the electrodes become contaminated, which causes the resistance at the machining gap to decline and increases the machining groove, with the result that the machining accuracy declines. In this case, it is conceivable to increase the pressure of the working fluid to promote the discharge of the machining sludge, but if the pressure of the working fluid is increased, the linear velocity of the working fluid between the wire electrode and the workpiece becomes high, so that there are cases where the working fluid is removed from side walls of the workpiece and the wire electrode, causing a hindrance to machining. Namely, if the working fluid is removed from the side walls of the workpiece and the wire electrode, the working electric current ceases to flow stably, so that there arises the problem of disconnection of the wire electrode. Thus, there is a limit to increasing the pressure of the working fluid.
Further, since the cross section of the conventional wire electrode is circular and the surface is smooth, the cooling efficiency based on heat transfer is low. Hence, in a case where a desired cooling effect cannot be obtained even if the working fluid is supplied at high speed, there have been cases where the machining speed has to be lowered to prevent the disconnection of the wire.
The present invention has been made to overcome the above-described problems, and its object is to obtain a wire electrical discharge machining apparatus which is capable of effectively promoting the cooling of the wire electrode and of improving the machining speed.
A wire electrical discharge machining apparatus according to a first aspect of the invention comprises: an upper guide and a lower guide which are respectively disposed above and below a workpiece and respectively incorporate wire guides for guiding the wire electrode; a pair of electric supply dies respectively provided in the upper guide and the lower guide and adapted to come into contact with and energize the wire electrode; an internal nozzle and an external nozzle which are provided in at least one of the upper guide and the lower guide; a first piping system for supplying a working fluid to the internal nozzle; a second piping system for supplying the working fluid to the external nozzle, the second piping system being independent of the first piping system; a first cooler for cooling the working fluid which is supplied to the internal nozzle through the first piping system; and a second cooler for cooling the working fluid which is supplied to the external nozzle through the second piping system, wherein the working fluid having lower temperature and higher pressure than the working fluid jetted out and supplied from the external nozzle is jetted out and supplied to the workpiece from the internal nozzle.
As for the wire electrical discharge machining apparatus according to a second aspect of the invention, the wire electrical discharge machining apparatus according to the first aspect of the invention further comprises: jetting means for jetting the working fluid toward the wire electrode to cause the wire electrode to be pressed against the electric supply die, wherein the working fluid supplied to the jetting means is the same as the working fluid supplied to the internal nozzle.
As for the wire electrical discharge machining apparatus according to a third aspect of the invention, in the wire electrical discharge machining apparatus according to the first aspect of the invention, an antifreeze solution is mixed in the working fluid which is supplied to the internal nozzle, the antifreeze solution is cooled by the cooler, supercooled working solution at a temperature of 0xc2x0 C. or lower is jetted out and supplied from the internal nozzle toward the workpiece.
The wire electrical discharge machining apparatus according to a fourth aspect of the invention comprises: an upper guide and a lower guide which are respectively disposed above and below the workpiece and respectively incorporate wire guides for guiding the wire electrode; a pair of electric supply dies respectively provided in the upper guide and the lower guide and adapted to come into contact with and energize the wire electrode; an internal nozzle and an external nozzle which are provided in at least one of the upper guide and the lower guide, the internal nozzle being adapted to jet out and supply high-pressure working fluid toward the workpiece, the external nozzle being adapted to jet out and supply toward the workpiece the working fluid having lower pressure than the high-pressure working fluid; a jet-stream generating means supported in such a manner as to be vertically movable around an outer peripheral portion of the wire guide, so as to guide the wire electrode from the upper guide to the lower guide by means of a jet stream; and an internal-nozzle driving means for supporting the internal nozzle in such a manner as to render the internal nozzle vertically movable, so as to drive the internal nozzle vertically through fluid pressure, wherein the internal nozzle is driven by the internal-nozzle driving means so as to drive the jet-stream generating means engaging with the internal nozzle.
The wire electrical discharge machining apparatus according to a fifth aspect of the invention comprises: an upper guide and a lower guide which are respectively disposed above and below the workpiece and respectively incorporate wire guides for guiding the wire electrode; a pair of electric supply dies respectively provided in the upper guide and the lower guide and adapted to come into contact with and energize the wire electrode; and an internal nozzle and an external nozzle which are provided in at least one of the upper guide and the lower guide, the internal nozzle being adapted to jet out and supply high-pressure working fluid toward the workpiece, the external nozzle being adapted to jet out and supply toward the workpiece the working fluid having lower pressure than the high-pressure working fluid, wherein a fin is provided on an outer peripheral portion of the wire electrode.
As for the wire electrical discharge machining apparatus according to a sixth aspect of the invention, in the wire electrical discharge machining apparatus according to the fifth aspect of the invention, the fin is inclined with respect to a central axis of the wire electrode.